This quotation comes following the Finches' eventful Christmas Day at the Landing. Once again, Scout has lost her temper and gotten into another fight--and again winning--a battle with a boy, her obnoxious Cousin Francis.
This time I split my knuckle to the bone on his front teeth. (Chapter 9)
Atticus and his brother Jack are discussing Scout's "hotheadedness," and Atticus is defending his daughter, explaining that Scout has made progress but that she still needs "assistance sometimes." Although Scout still has a long way to go, and Atticus knows that the upcoming months before the Tom Robinson trial will be even worse for her, he also realizes that she is trying her best to hold her temper when her pride is at stake. Atticus may not be completely happy with the results, but he is pleased with her effort.
"... the answer is she knows I knows she tries. That's what makes the difference." (Chapter 9)